1. Field of the Invention
The present invention pertains generally to waveform sequential sampling techniques and apparatus.
2. Description of the Background
Measuring an input waveform signal is limited by how fast direct sampling measurements can be taken or by how well a sampling technique can be synchronized to the input waveform and resolve fractions of a clock cycle.
Generally, the highest frequency that can be measured by any type of sampling is one-half of the frequency of the sampling. This is called the Nyquist Frequency. In any digital processing system, there is a minimum allowable sampling frequency called the Nyquist Frequency and it is specified by the Sampling Theorem, summarized as follows: When digitizing an analog signal, the sampling rate must be at least twice as great as the highest frequency component (fo) in the spectrum of the sampled signal. Frequency components higher than fo will "alias" down into the frequency range below fo and interfere with the accurate representation of the sampled signal. To avoid signal distortion caused by aliasing, the effective sample interval must meet the Nyquist criterion of 1/(2fo). In direct sampling, the effective sample interval is the actual time between measurements. Therefore, for a 20 microseconds sample interval, the maximum signal frequency which can be measured without distortion is 25 KHz. Also, if higher frequencies are present, then a low-pass filter of bandwidth fo and less should be inserted in the input signal path.
Sequential sampling (also known as sub-sampling) techniques can reduce the effective sample interval and thus, allow measurement of higher frequency waveforms or provide greater resolution and accuracy for a given frequency waveform. In order to do sequential sampling, the waveform to be measured must be repetitive since the period is reconstructed in several passes of the waveform. For sequential sampling, the effective sample interval is the time between samples of the reconstructed waveform. For instance, if the effective sampling period is 35 nanoseconds then a frequency of 14 MHz can be measured. Also, sequential sampling can be done on waveforms which would be within the Nyquist criterion for direct sampling, but a higher resolution and greater accuracy is desired.
In order to perform sequential sampling, the measurement sample taken must be referenced to a known point on the input waveform. Sequential sampling gives a series of measurements. Since these measurements are taken over multiple periods of the input waveform, the order in which the measurements are taken is not necessarily an indication of the order of the measurements on the waveform. Generally, the measurements must be rearranged to reflect the true position on the input waveform. This ordering is done by using a reference on the input signal. All measurements are taken with respect to this known reference value. A trigger is established on this reference value. When the input waveform attains the reference value, this causes a trigger. Measurements taken, are taken from a given known delay time from the trigger. This delay time, from when the trigger occurs to when the measurement is taken, is measured. The accuracy of reconstructing the input waveform is a function of how well this delay time can be measured. In this way, signal value and time can be correlated and the signal reconstructed. The trigger can be established by the level and slope of the input signal. For example, a trigger would occur at the raising zero voltage crossing of a waveform. The delay time can be a predetermined number of clock cycles. For example, a first measurement would be taken four (4) clock cycles after the trigger, the second measurement taken five (5) clock cycles after the trigger, and so forth until enough measurements were taken to resolve the input waveform. In this example, the effective sampling interval would be one clock cycle.
As seen above, after each measurement, the predetermined delay is changed so as to effectively take a measurement on a different part of the input waveform. Often, the delay would be incremented by one clock cycle, and therefore, for a 10 MHz clock, the effective sampling interval would be 100 nanoseconds and the Nyquist Frequency would be 5 MHz.
The predetermined delay, for the most part, is counted by an internal clock. However, since the trigger can occur anytime during a clock cycle, the precision of the predetermined delay will depend on the length of the clock cycle and the particular technique chosen to interpolate between the clock cycles to determine when the trigger occurred.
Sequential sampling techniques handle the problem of the delay between a trigger and a clock edge in various ways. One method is to do nothing about this delay and calculate the uncertainty into the resolution, thus reducing the resolution of the input waveform reconstruction. It is likely that the input signal to be synchronized will come in-between a clock pulse. A clock pulse on a 10 MHz. clock comes every 20 MHz or 50 nanoseconds. If one were to wait until a clock pulse in order to synchronize the clock to the waveform, the synchronization could be displaced from 0 to 50 nanoseconds: a maximum error of 50 nanoseconds. For some purposes, this error would be adequate and is tolerated. For accurate measurements on high speed waveforms, this error is intolerable.
Preferably, sequential sampling techniques interpolate between the clock cycles. One way is to use a phase triggerable phase lock loop. This method is expensive and requires complicated circuitry. Also, another method is to use delay line circuitry. This requires dedicated precisely controlled circuitry. A common method is to use a ramp interpolator. A ramp interpolator is usually in the form of a constant current source, a capacitor, a comparator, and the appropriate switches. On a given signal, the constant known current source charges the capacitor. At the end of the duration that is to be measured, the current source is disconnected from the capacitor. The voltage on the capacitor is equivalent to the duration to be measured. The duration can be found by discharging the capacitor at a known current and using that time interval as an equivalence to the first duration or the voltage could be measured or compared and the delay time calculated from known calibration points or a look-up table.
Another way of using a ramp interpolator is to interpolate between clock cycles and use the measured duration to add on another duration to complete one clock cycle or a predetermined number of clock cycles. This allows synchronization of the clock to an unknown input waveform. The ramp interpolator will measure the duration between the input trigger and the next clock pulse, and add on the complement duration so the clock becomes synchronized to the input waveform. The single cycle ramp interpolator then completely discharges and waits for another trigger before measuring another duration.
Sub-sampling techniques often require 500 or more samples in order to adequately reconstruct the input waveform. Using a single cycle ramp interpolator takes a relatively long time since it will have to wait for 500 triggers or 500 periods of the input waveform.
Also, single cycle ramp interpolators are generally limited to changing the predetermined delay between the trigger and the measurement by one clock cycle at a time, thus limiting the effective sample rate to one clock cycle. Therefore, a 10 MHz clock would have an effective sample rate of 100 nanoseconds and would be limited to measuring frequencies of less than 5 MHz. The problem is to reduce the time needed to reconstruct an input waveform and decrease the effective sample interval.